The present invention relates to a resin systems, and more specifically to epoxy resin-hardener systems, and products made thereof.
Epoxy resins are among the most versatile polymeric materials. Examples of their applications are coatings, adhesives, casting compositions, molding compositions, potting compositions for encapsulating electronic components, laminates and base material for printed circuits, and also matrix resins for fiber-reinforced plastics.
The conversion of monomeric or oligomeric epoxy resins into polymers requires reaction partners which are termed hardeners. Depending on the type of hardener, the curing reaction takes place at temperatures around room temperature, or at low temperatures (known as cold curing) or at elevated temperatures (known as hot curing).
For curing epoxy resins at low temperatures for industrial applications it is predominantly only aliphatic primary or secondary amines and polyaminoamides which are used, and less frequently polythiols or specific ionic catalysts. All unmodified amines have an alkaline to strongly alkaline reaction. liquid amines, in particular the aliphatic and cycloaliphatic amines, can cause skin damage extending to corrosive burns. Another disadvantage is the high volatility of liquid amines. A great disadvantage of cold curing of epoxy resins using the abovementioned hardeners is the low heat resistance and chemicals resistance of the resultant products.
To increase heat resistance, solvent resistance and chemicals resistance it is necessary to complete the curing of epoxy resins at elevated temperatures, by hot-curing using aromatic or cycloaliphatic amines, carboxylic anhydrides, polyphenols, novolaks or using latent hardeners.
It is known from Houben-Weyl, Methoden der Organischen Chemie [Methods in Organic Chemistry], Vol. E20, Makromolekulare Stoffe [Macromolecular materials], Georg Thieme Verlag Stuttgart, 1987, p. 1959 that, in particular in the case of bisphenol A resins, the curing of epoxy resins with cyclic dicarboxylic anhydrides or with tetracarboxylic bisanhydrides gives cured products with excellent electrical insulation properties and with good heat resistance. A further advantage is that curing with anhydrides, unlike curing with amines, is not significantly exothermic. However, a disadvantage is that curing temperatures of at least 120-150xc2x0 C. are always required, and even then some hours are needed for curing. Even at these temperatures, the crosslinking reaction is still so slow that it is generally essential to use accelerators. However, experience has shown that the use of curing accelerators can lead to loss of quality in the cured resin systems.
U.S. Pat. No. 5,629,379 describes a cured epoxy resin system made from a mixture of four components which gels at a temperature between 80 and 120xc2x0 C. and is cured at temperatures between 200 and 300xc2x0 C. Besides the epoxy resin component and the anhydride hardener component, the mixture comprises in particular an additional hardener component, and also a curing accelerator component.
U.S. Pat. No. 4,559,272 describes a process for potting an electrical component by impregnating the electrical component with a hot-curing composition made from a polyglycidic aromatic amine, a polycarboxylated carboxylic anhydride and a curing accelerator, and then curing the composition. As disclosed in that publication, the curing accelerator is in particular added in order to achieve a low curing temperature. The gel point disclosed in the examples is about 100xc2x0 C. In U.S. Pat. No. 4,595,623, the same applicant describes a fiber-reinforced, syntactic foam composite material which as matrix likewise has the abovementioned hot-curing composition.
DE 26 50 746 and U.S. Pat. No. 4,002,599 describe a room-temperature-curing epoxy resin composition which comprises a mixture of a polyglycidylarninophenol epoxy resin and a biphenyl anhydride. All of the examples described here are in particular based on the curing of triglycidyl-p-aminophenol (TGpAP) with benzophenone-3,3xe2x80x2,4,4xe2x80x2-tetracarboxylic dianhydride (BTDA) as sole hardener, or with hardener mixtures made from BTDA and maleic anhydride (MA).
However, the TGpAP-BTDA system described in DE 26 50 746 and U.S. Pat. No. 4,002,599 proves, in particular in industrial or practical use, to be disadvantageous and highly problematic, since the BTDA is extremely difficult to dissolve in TGpAP. The inventors therefore recommend an extreme degree of mixing, e.g. three hours in a ball mill. Another recommendation is that the BTDA should be very finely pulverized or treated with high shear forces on a three-roll mill. However, these lengthy procedures can in particular bring about hydrolysis of the anhydride, and in addition the dissolution of BTDA in TGpAP generally remains incomplete. Further to this, additional production steps lead to prolonged production times and thus to increased production costs.
The publication by J. E. O""Connor and J. A. Graham, the inventor in DE 26 50 746 and U.S. Pat. No. 4,002,599, in Adhesives Ages 21/7 (July 1978), pp. 20-23 entitled xe2x80x9cEpoxy with Low-Temperature Cure and High-Temperature Properties Developedxe2x80x9d includes a further description and commentary of the invention disclosed in DE 26 50 746 and U.S. Pat. No. 4,002,599. As can be found in lines 20 et seq. of the 3rd column on p. 20, the authors themselves in their invention expressly describe the TGpAP-BTDA system as an exception, as follows: xe2x80x9cThe TGpAP-BTDA system appears to be the xe2x80x98exception to the rulexe2x80x99 regarding BTDA as a hardener for epoxy resinsxe2x80x9d.
There is therefore a demand for epoxy resin-hardener systems which can cure at a low temperature and give products which have increased heat resistance, chemicals resistance and solvent resistance. Examples of potential applications would be adhesives, matrix resins for fiber composite materials and repair resins for components, in cases where the use of high temperatures is not permissible. Other applications would be casting compounds and potting compounds, specifically for encapsulating large electronic components, in cases where the curing can be completed at low temperature, with little exothermic effect, and therefore with a considerable energy saving, another advantage here being that the products produced have less internal stress.
It is an object of the present invention, therefore, to provide a resin system which can be crosslinked at low temperature, in particular at room temperature, and preferably without using a curing accelerator.
Another object of the present invention is to provide a resin system whose solubility behavior in particular, and particularly that of the hardener component, is better than that of known systems.
A further object of the present invention is to provide a process for producing an insulated electrical conductor, the insulation of which can take place at low temperatures, in particular room temperature.
The term xe2x80x9ccrosslinkabilityxe2x80x9d here is to be taken as meaning the capability of a polymerizable system to assume an irreversible state.
Surprisingly, it has now been found that the system described by Graham and O""Connor is not an exception but merely one example of the generally applicable principle of the curing of epoxy resins by cyclic anhydrides at low temperatures. However, a precondition is that the individual polyfunctional epoxy resins, or the various polyfunctional epoxy resins which can be used in mixtures, comprise an epoxy resin having at least one aminoglycidyl group. According to the invention, a wide variety of aromatic, aliphatic, cycloaliphatic or heterocyclic acid anhydrides is suitable for the cold curing of epoxy resins, preferably giving products with increased heat resistance. The authors Graham and O""Connor did not recognize the generally applicable principle within their invention.
Surprisingly, it has now been found that there are numerous combinations of cyclic anhydrides and aminoglycidyl compounds which have markedly better solubility behavior than the system described by Graham and O""Connor and which give homogeneous and transparent moldings, cf. Table 3.
The epoxy resins which can be used in the present invention have on average more than one epoxy group per molecule, and at least one of these groups must be present in the form of an aminoglycidyl compound. Particularly suitable aminoglycidyl compounds of this invention are N,N-diglycidylaniline, N,N-diglycidyltoluidine, N,N,Nxe2x80x2, Nxe2x80x2-tetraglycidyl-1,3-diaminobenzene, N,N,Nxe2x80x2,Nxe2x80x2-tetraglycidyl-1,4-diaminobenzene, N,N,Nxe2x80x2, Nxe2x80x2-tetraglycidylxylylenediamine, N,N,Nxe2x80x2,Nxe2x80x2-tetraglycidyl4,4xe2x80x2-diaminodiphenylmethane, N,N,Nxe2x80x2,Nxe2x80x2-tetraglycidyl-3,3xe2x80x2-diethyl4,4xe2x80x2-diaminodiphenylmethane, N,N,Nxe2x80x2, Nxe2x80x2-tetraglycidyl-3,3xe2x80x2-diaminodiphenyl sulfone, N,Nxe2x80x2-dimethyl-N,Nxe2x80x2-diglycidyl-4,4xe2x80x2-diaminodiphenylmethane, N,N,Nxe2x80x2,Nxe2x80x2-tetraglycidyl-xcex1,xcex1xe2x80x2-bis(4-aminophenyl)-p-diisopropylbenzene and N,N,Nxe2x80x2,Nxe2x80x2-tetraglycidyl-xcex1,xcex1xe2x80x2-bis(3,5-dimethyl-4-aminophenyl)-p-diisopropylbenzene. Other particularly preferred aminoglycidyl compounds of the formula (I) are given in Table 6 with tradename, manufacturer and structural chemical formula.
Particularly suitable polyglycidyl compounds of aminophenols are O,N, N-triglycidyl-4-aminophenol, O,N,N-triglycidyl-3-aminophenol and 2,2-(N,N-diglycidyl-4-aminophenyl-1,4xe2x80x2-glycidyloxyphenyl)propane. Other aminoglycidyl compounds which may be used according to the invention are described in Houben-Weyl, Methoden der Organischen Chemie, Vol. E20, Makromolekulare Stoffe, Georg Thieme Verlag Stuttgart, 1987, pp. 1926-1928. The tri- and tetrafunctional aminoglycidyl compounds may be prepared by the processes described in U.S. Pat. No. 2,884,406, U.S. Pat. No. 2,921,037 and U.S. Pat. No. 2,951,822, for example, or else by those described in EP 148 117.
Other polyepoxides may be used concommitantly with the aminoglycidyl compounds, and their proportion in the epoxy resin mixture may be from 1 to not more than 75 mol %, preferably from 10 to 50 mol %. These other polyepoxides may be aliphatic, cycloaliphatic, aromatic or heterocyclic, and may also have substituents, such as halogens, hydroxyl, ethers radicals or other radicals. They are generally based on the polyhydric phenols which are known and described in detail in the literature, i.e. bisphenol A, bisphenol F and bisphenol S, on epoxidized phenol novolaks or on epoxidized cresol novolaks, or on cycloaliphatic epoxy resins. Examples of aliphatic epoxy resins are epoxyalkanes, diglycidyl ethers of diols, and also cis/trans-1,4-cyclohexanedimethanol diglycidyl ethers. Examples of cycloaliphatic epoxy resins are cyclohexene oxide, 4-vinyl-1-cyclohexene diepoxide and 3,4-epoxycyclohexylmethyl 3,4epoxycyclohexanecarboxylate.
Suitable hardeners are cyclic anhydrides of aromatic, aliphatic, cycloaliphatic or heterocyclic polycarboxylic acids. Particularly suitable anhydrides of aromatic polycarboxylic acids can be described by the formulae I to III, where R1, R2, R3 and R4 may be hydrogen or substituents such as halo groups, alkyl groups, alkoxy groups or nitro groups, etc. Z may be xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94CH2xe2x80x94, or another alkylene radical, or else an oxoalkylene radical, where n=0 or 1. 
Examples of anhydrides having the structures I to III are: phthalic anhydride, 4-methyl-phthalic anhydride, 3,6- and 4,5-dichlorophthalic anhydride, 3,6-difluorophthalic anhydride, tetrabromo-, tetrachloro- and tetrafluorophthalic anhydride, 3- and 4-nitro-phthalic anhydride, benzene-1,2,4,5-tetracarboxylic dianhydride (pyromellitic dianhydride), 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic dianhydride, 2,2-bis(3,4-dicarboxy-phenyl)propane dianhydride, 2,2-bis(2,3-dicarboxyphenyl)propane dianhydride, 2,2-bis-(3,4-benzenedicarboxylic anhydride)perfluoropropane, bis(3,4-dicarboxyphenyl) ether dianhydride, bis(3,4-dicarboxyphenyl)methane dianhydride, 2,2xe2x80x2, 3,3xe2x80x2-biphenyltetracarboxylic dianhydride and 3,3xe2x80x2,4,4xe2x80x2-biphenyltetracarboxylic dianhydride. The aromatic carboxylic anhydrides may contain additional functional groups. Examples of such compounds are: benzene-1,2,4-tricarboxylic anhydride (trimellitic anhydride), 3-hydroxyphthalic anhydride, 3- and 4-maleimidophthalic anhydride, and also the cyclic anhydride of 2-sulfobenzoic acid. Other suitable compounds are derivatives of trimellitic anhydride, e.g. the bis(trimellitic anhydride) of neopentyl glycol.
Besides the anhydrides described under I to III, aromatic carboxylic anhydrides having condensed ring systems are also suitable: 1,8-naphthalic anhydride, naphthalene-1,4,5,8-tetracarboxylic dianhydride, naphthalene-1,2,5,6-tetracarboxylic dianhydride, naphthalene-3,4,5,8-tetracarboxylic dianhydride, naphthalene-2,3,6,7-tetracarboxylic dianhydride, 3,4,9,10-perylenetetracarboxylic dianhydride, and their halo and nitro derivatives.
The aliphatic dicarboxylic anhydride particularly suitable and preferred in this invention is maleic anhydride. It can be used as sole hardener for aminoglycidyl resins and for their mixtures with other epoxy resins. Maleic anhydride has very good solubility in the aminoglycidyl resins. In mixtures with other anhydrides, maleic anhydride acts as solubilizer and increases the rate of dissolution. The cured resins have excellent heat resistance and high glass transition temperatures. Other suitable anhydrides are derivatives of maleic anhydride, e.g. methyl- and dimethylmaleic anhydride, phenyl- and diphenylmaleic anhydride, bromomaleic anhydride, dicholoromaleic anhydride and the like.
Some cycloaliphatic dicarboxylic anhydrides which can be used according to the invention have the advantage of being liquid or low-melting, for example cis-cyclohexane-1,2-dicarboxylic anhydride, 4-methylcyclohexane-1,2-dicarboxylic anhydride, methylcyclo-hexene-4,5-dicarboxylic anhydrides and bicyclic compounds, such as methyl-5-norbornene-2,3-dicarboxylic anhydride and its isomeric mixtures (commercially available as NADIC methyl anhydride). Other suitable mono- or bicyclic cycloaliphatic dicarboxylic anhydrides are 1,2,3,4-cyclobutanetetracarboxylic dianhydride, cis-1,2,3,4-cyclopentanetetracarboxylic dianhydride, trans-cyclohexane-1,2-dicarboxylic anhydride, 1-cyclopentene-1,2-dicarboxylic anhydride, cis-4-cyclohexene-1,2-dicarboxylic anhydride, cis-5-norbornene-endo-2,3-dicarboxylic anhydride (NADIC anhydride), bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride and 1,4,5,6,7,7-hexachloro-5-norbornene-2,3-dicarboxylic anhydride (HET anhydride).
Examples of heterocyclic carboxylic anhydrides which may be used according to the invention are pyridine-2,3-dicarboxylic anhydride, pyridine-3,4-dicarboxylic anhydride, pyrazine-2,3-dicarboxylic anhydride, pyrazine-2,3,5,6-tetracarboxylic dianhydride, thiophene-2,3,4,5-tetracarboxylic dianhydride, tetrahydrofuran-2,3,4,5-tetracarboxylic dianhydride, isatoic anhydride and N-methylisatoic anhydride.
Other cyclic anhydrides particularly preferred for the present invention are given in Table 7, with the abbreviations used here and the structural chemical formula.
Together with the mixtures made from aminoglycidyl compounds with cyclic carboxylic anhydrides, comcomitant use may be made of lactones. The lactones are liquid or readily fusible, have low viscosities, and improve the solubility of the acid anhydrides in the mixtures, and act as reactive diluents. Mixtures made from epoxy resins with lactones have been described in the literature. However, they have hitherto been curable at low temperatures only by amines, cf. BE 617.540, while the curing of the epoxide-lactone mixtures by acid anhydrides again requires the usual curing temperatures of from 120 to 180xc2x0 C., cf. U.S. Pat. No. 3.222.321.
Examples of lactones which according to the invention can be used concomitantly in the low-temperature curing of aminoglycidyl compounds with cyclic anhydrides are xcex2-propiolactone, xcex2-butyrolactone, xcex3-butyrolactone, xcex3-valerolactone and xcex5-caprolactone and also derivatives of these, e.g. 2-acetyl-xcex3-butyrolacetone. The proportion of the lactones in the epoxy resin mixture may be from 0.1 to 1.5 mol per epoxy group equivalent.
The definition and calculation of the equivalents used in this text, whether epoxy equivalent of the first component or anhydride equivalent of the second component or the like, are known to the skilled worker and require no further explanation.
The cyclic carboxylic anhydride used as hardener is used at concentrations of from 0.2 to 1.2 equivalents of anhydride groups per epoxy equivalent. It is advantageous for the anhydride particles to be as fine as possible and to be introduced into the liquid epoxy resin mixture using mixing assemblies with vigorous action. If the anhydrides or the mixtures of these are liquid or readily fusible, they form the initial charge to which the epoxy resins are then added. Short mixing times using mixing or dispersion units running at high rotation rates are preferred to assemblies which run more slowly, e.g. roll mills or ball mills. It is also possible to use ultrasound or microwaves to obtain rapid and thorough mixing.
The curable mixtures of the invention may also comprise plasticizers or elasticizers, or additives, such as extenders, fillers, reinforcing fibers or flame retardants.
According to the invention, the curing may preferably take place at low temperatures, e.g. from 0 to 90xc2x0 C. It is in principle possible to cure the mixtures of the invention at room temperature, achieving a degree of completion of curing of more than 90%, followed by thermal analysis methods (DSC), within a period of from some days to a few weeks. Higher temperatures, from 30 to not more than 90xc2x0 C., are recommended if the epoxy resin mixtures remain solid or are highly viscous at room temperature, or if the dissolution of solid anhydrides in the epoxy resin mixture is to be accelerated. If desired, the curing may also be undertaken in two stages, by beginning to cure the curable mixture at low temperature and postcuring (annealing) the same at an elevated temperature. This method is preferred if the relatively long times for completion of curing at room temperature are to be shortened. One way is to allow the curable mixtures of the invention to cure for from 20 to 24 hours at room temperature, followed by postcuring for from 1 to 2 hours at temperatures of from 50 to 90xc2x0 C. The resultant degree of completion of curing (DSC) is 97% or above.
Examples of uses of the curable compositions of the invention are laminating resins, saturating resins and casting resins, coatings, potting compounds and insulating compositions for electrical engineering, but particularly adhesives which give increased heat resistance. the increased heat resistance of the curable compositions of the invention is demonstrated using thermal analysis in the form of thermogravimetric analysis (TGA) and dynamic-mechanical analysis (DMA).
In all of the examples of applications, the concentration of the cyclic carboxylic anhydrides used as hardeners, individually or in mixtures, is always 0.64 in total anhydride group equivalents per epoxy equivalent, unless otherwise stated. If maleic anhydride is used in a mixture with other anhydrides, the ratio by weight of maleic anhydride to the second anhydride is always 3:2.
All of the resin-hardener mixtures are dispersed at 13500 rpm for 4 minutes using an Ultraturrax disperser and then cast to give test specimens for DMA. A second mix serves for production of cast articles of 2 cm thickness for visual assessment of homogeneity. To improve the thoroughness of mixing, highly viscous mixtures are subjected to prior heating for a few minutes to 30-50xc2x0 C. The conditions for completion of curing are 24 h aging at room temperature followed by one hour of postcuring at 90xc2x0 C. The viscosities of the mixtures are determined using a cone-and-plate viscometer at 25xc2x0 C. The DMA specimens are tested using a heating rate of 3xc2x0 C./min at a fixed frequency of 1 Hz, using an amplitude of 0.2 mm. The damping maximum (tan xcex4) serves to establish the glass transition temperature (Tg).
Table 1 lists the results of TGA together with the onset point for thermal decomposition and the residue at 600xc2x0 C. It can be seen that the onset point of decomposition for all of the epoxides whose curing had been completed using anhydrides is much higher than those for completion of curing with DETA. Table 2 has the results from the viscosity measurements and the glass transition temperatures from DMA.
Table 3 lists the results of visual testing for homogeneity on fully cured test specimens of 2 cm thickness. The evaluation scale extends from 0 for absolutely homogeneous, transparent specimens through xe2x88x921 for slight haze, xe2x88x922 to xe2x88x923 for increasing haze, up to xe2x88x924 and xe2x88x925 for specimens in which, respectively, there is slight or extensive presence of a sediment of undissolved solids. The results show that many of the systems according to the invention have considerably better homogeneity, i.e. better solubility of the anhydrides in the epoxy resins, than the system described by Graham.
The results in Table 4 prove that even mixtures of bisphenol A epoxy resins with aminoglycidyl compounds can be fully cured by anhydrides at low temperatures. However, the proportion of the aminoglycidyl compound in the mixture should not be below 25 mol %.
The following abbreviations are utilized in Tables 1 to 5, and are listed here in alphabetical order; further explanations can be found in Tables 6 and 7.